Cell reference must work with formula output, not formula itself

In a timesheet, I want the result of a formula to depend on the value of a referenced cell, not the formula itself. To clean up my timesheet, I want Excel to leave the cell "hours/day" blank if that day is either Saturday or Sunday.

Column B has the date in format dd/mm/yyyy
Column A shows the day of the week (in Dutch) Monday, Tuesday, etc. (abbreviated).
Column C has the time I started working,
Column D shows the time I finished working.
Column E has the amount of time I took for lunch.

Column F has this formula:
=IF(A4="za";"";IF(A4="zo";"";((D4-C4-E4)*24)))

However, even though A4 displays 'za' (the Dutch equivalent for 'Sat') it still does not display a blank cell. I suspect the reason is because my formula refers to the relative value of cell A4, not the absolute value (the result of the formula in A4, rather than the formula itself).

How can I make sure that for Saturdays and Sundays the fields are left blank?